In a crucible furnace, the metal to be heated is placed in a silicon carbide crucible supported on a pedestal within a furnace which is commonly heated by gas-fired or oil-fired burners acting on the crucible. The furnace has a steel shell with an internal lining. The working inner face (hotface) of the shell lining is subjected to the maximum temperature in the furnace and is spaced from the crucible a predetermined distance. Hence, since the maximum size of crucible for a given furnace is limited by the thickness of the lining, it is desirable to minimize the lining thickness.
In the handling of molten metal, it is advantageous to have a durable lining with the physical ability to withstand the conditions at the hotface for long life while also insulating against heat loss from the vessel. Unfortunately, strong refractory materials generally do not have the heat resistivity required to meet efficient thermal requirements.
In the past, it has been common in the crucible furnace art to use ceramic fiber liner material offering reactively fast installation with no cure-out and good insulation quality. The disadvantages are relatively poor heat retention and resistance to burner erosion, and short life due to poor resistance to metal spills. Various castable refractories have been used having the advantage of being inexpensive but having as disadvantages poor resistance to erosion and metal attack if formulated for a good insulation characteristic and poor heat retention if formulated for strength. Also, the castable refractories have been criticized as being messy and unduly time-consuming to apply and as requiring long cure-outs. Various plastics have also been used to give strong mechanical strength to resist erosion and good resistance to metal wetting, but these too have had relatively poor heat retention and insulation characteristics, have been overly time-consuming to install, and have involved extensive cure-out schedules.